1. Field of the Invention
The invention relates to a drilling tool.
2. Background and Relevant Art
Drilling tools are known which are rotatable about a tool axis and which, on their face side, have two or three main cutting edges which merge in the inward direction into a chisel edge. The chisel edge runs in a central region or core region of the drilling tool on the face side thereof. Contrary to its name, the chisel edge of a drilling tool usually primarily has not a cutting but rather a scraping action. It is usually perpendicular to the drilling or feed direction, and increases the required working pressure. The feed force for the chisel edge may amount to approximately ⅓ of the entire feed force for the drilling tool. Furthermore, there is the risk of a so-called “runaway”, that is to say a lateral positional displacement upon the start of drilling. The self-centering capability of such drilling tools is therefore not particularly good.
To reduce said disadvantages, drilling tools, in particular drilling tools with straight main cutting edges, are often provided with a so-called point thinning, in particular with a crossed point thinning or a so-called S-shaped point thinning, in order to shorten the chisel edge. A disadvantage of said point thinning shapes is however that the self-centering capability of such drilling tools is not good. The reason for this is that the angle between the point-thinned chisel edge, which is also referred to as the residual chisel edge, and the chisel edge part over the central region is highly obtuse. Said angle usually lies in a range from 130° to 160°.
It has therefore already been attempted to increase the residual chisel edge angle, that is to say the angle between the residual chisel edge and a straight connecting line of the outer ends of the main cutting edges, the so-called cutting edge corners, from the usual 55° to approximately 70°. This has the disadvantage, however, that the clearance angle between the drilling tool and machine surface becomes considerably smaller in the central region, and the drilling tool no longer moves freely in the central region.
Another possibility for improving the self-centering capability is to reduce the chisel edge angle, that is to say the angle between the non-point-thinned chisel edge and the connecting line of the cutting edge corners, to for example 10°. This is however only possible in drilling tools with a concavely curved main cutting edge. In drills with a straight or convexly curved main cutting edge, the reduction in said chisel edge angle would result in the point thinning being drawn into the region of the main cutting edge. This would have the disadvantage of the rake angle in said region becoming too small.
Patent document DE 83 531 and the additional patent 84 413 propose a cylindrical drill with a straight or helically coiled longitudinal groove which bears sharp-edged longitudinal ribs on the surfaces pointing towards the cutting edges.
DE 199 83 068 T1 relates to a drilling head with a concave chisel edge which merges with secondary cutting edges of the two bevels via the axially outermost extremities, which are spaced apart from the axis, of said secondary cutting edges.